1. Field of the Invention
The invention relates to a solid-state image sensor to be employed in a visible radiation region, and a method of fabricating the same.
2. Description of the Related Art
As a solid-state image sensor including a photodetector having electrodes in a photoelectric conversion region and detecting a light passing through the electrode, there are a charge coupled device (CCD) which is one-dimensionally fabricated, a charge injection device (CID) image sensor, a time delay and integration (TDI) operation CCD image sensor, a frame transfer type CCD image sensor, and a full-frame type CCD image sensor.
In a CID image sensor, photoelectric conversion is carried out at a portion below a gate electrode of a MOS capacitor. In a one-dimensional CCD, TDI operation CCD image sensor, a frame transfer type CCD image sensor, and a full-frame type CCD image sensor, a CCD having a function of electronic scanning is employed also as a photodetector.
FIG. 1 illustrates a basic structure of CID image sensor, and FIGS. 2A to 2C illustrate an operation of a unit pixel of a CID image sensor. A unit pixel 13 illustrated in FIG. 1 is constituted of two charge-coupled MOS capacitors, as illustrated in FIGS. 2A to 2C. In a CID image sensor, a plurality of unit pixels 13 is two-dimensionally arranged. Each one of the unit pixels 13 is electrically connected with one of horizontal selection lines 16 and further with one of vertical selection lines 17. All the horizontal selection lines 16 are connected to a vertical shift register 14, and all the vertical selection lines 17 are connected to a horizontal shift register 15. An individual unit pixel among a plurality of the unit pixels 13 is accessible by means of a horizontal selection pulse applied to one of the horizontal selection lines 16 and a vertical selection pulse applied to one of the vertical selection lines 17. Hereinbelow, an operation for selecting one of the unit pixels 13 is explained with reference to FIGS. 2A to 2C.
As illustrated in FIG. 2A, the unit pixel 13 is comprised of a p-type Si substrate 18 which is grounded, an oxide film 21 formed on the substrate 18, a first gate electrode 19 formed on the oxide film 21 and electrically connected to the horizontal selection line 16, and a second gate electrode 20 formed partially on the oxide film 21 and partially on the first gate electrode 19 with the oxide film 21 sandwiched therebetween, and electrically connected to the vertical selection line 17.
FIG. 2A illustrates a condition where high level is applied to the horizontal selection line 16 and low level is applied to the vertical selection line 17. There is formed a potential well W in the p-type silicon substrate 18 below the first gate electrode 19 electrically connected to the horizontal selection line 16. Signal charges generated in the p-type Si substrate 18 by means of a light h .upsilon. passing through the first and/or second gate electrodes 19 and 20 and incident on the substrate 18 are accumulated in the potential well W.
FIG. 2B illustrates a condition where low level is applied to the horizontal selection line 16 and high level is applied to the vertical selection line 17. In this case, a potential well W is formed in the p-type silicon substrate 18 below the second gate electrode 20 electrically connected to the vertical selection line 17. Signal charges generated by an incident light h .upsilon. are accumulated in the potential well W.
As mentioned above, if at least one of MOS capacitors electrically connected to the horizontal and vertical selection lines 16 and 17 is in high level, signal charges are accumulated in one or both of the potential well(s) W formed below the first and second gate electrodes 19 and 20, or merely move between the two MOS capacitors, and do not flow out of the substrate 18.
To the contrary, if low level is applied to both horizontal and vertical selection lines 16 and 17, the potential well W ceases to exist in a pixel located at an intersection of the horizontal and vertical selection lines 16 and 17, as illustrated in FIG. 2C. Hence, signal charges become excessive minority carriers, and resultingly are swept towards the inside of the substrate 18. The minority carriers recombine with majority carriers to thereby generate a recombination current Is running between the ground and the substrate 18. Image signals are produced, utilizing the fact that the thus generated recombination current Is is in proportion to signal charges.
Since basic photoelectric conversion mechanism is common in all image sensors including CCD itself as a photodetector, hereinbelow is explained the photoelectric conversion mechanism of a frame transfer type CCD image sensor as an example.
FIG. 3 is a plan view of a frame transfer type CCD image sensor, and FIG. 4 is a cross-sectional view of an image section of the frame transfer type CCD image sensor illustrated in FIG. 3. In the illustrated frame transfer type CCD image sensor, a vertical CCD is of the three-phase drive type. Channel stop regions 8 vertically divide a charge transfer channel into pixel rows in the same number as the number of horizontal pixels. Vertical CCDs are arranged perpendicularly to the channel stop regions 8. The vertical CCDs are grouped into upper and lower groups. The upper group of the vertical CCDs forms an image section 22 for receiving a light therein, whereas the lower group of the vertical CCDs forms a storage section 23 for temporarily storing signal charges therein. The storage section 23 is connected at a termination end thereof with a horizontal CCD 9. The horizontal CCD 9 receives signal charges of a line at a time from the storage section 23, and carries them to an output section 10. The output section 10 carries out detecting of signal charges and converting the thus detected charges into voltage, and then emits the signal voltage as an output. A thin insulating film (not illustrated) is formed over the image sensor, and a metal film (not illustrated) is formed on the insulating film in order to prevent light from entering the storage section 23 and the horizontal CCD 9.
With reference to FIG. 4, the image section of the frame transfer type CCD image sensor includes a p-type Si substrate 18, a gate oxide film 4 formed on the p-type Si substrate 18, and a plurality of vertical CCD electrodes 26 arranged on the gate oxide film 4. One of drive signals I.sub..PHI.1 to I.sub..PHI.3 is applied to every three vertical CCD electrodes 26.
As illustrated in FIG. 4, a single pixel 27 corresponds to three vertical CCD electrodes 26. Herein, a single pixel means a region surrounded by the vertical CCD electrodes 26 in the number associated with the number of phases of drive signals 24 constituting one stage of CCD and the channel stop regions 8. There are formed potential wells W below the vertical CCD electrodes 26 to which the drive signals I.sub..PHI.1 is applied, that is, positive charges [+] are applied. When a light having an energy of h .upsilon. is incident on a light receiving region of the image section 22, excited signal charges are collected in the potential well W below one of the three vertical CCD electrodes 26 corresponding to a single pixel 27.
When a charge accumulation period for converting a light signal into signal a charge has finished, drive signals 24 for driving the vertical CCDs in the image section 22 and drive signals 25 for driving the vertical CCDs in the storage section 23 become high-speed transfer pulses, and thus charges in all pixels in the image section 22 are immediately transferred to and stored in the storage section 23.
Thereafter, the image section 22 starts again accumulating light signal charges in each of the pixels. While the image section 22 is accumulating light signal charges, the signal charges having been stored in the storage section 23 are downwardly transferred line by line, and then are transferred to the output section 10 through the horizontal CCD 9. First, signal charges stored in the bottom line are all transmitted to the horizontal CCD 9, and then transferred horizontally to the output section 10 at a high clock frequency. The thus transferred signal charges are read out of the output section 10 as time sequential signals. Since the next signal charges have already been transferred down to the bottom line at this time, the next signal charges are transferred into the horizontal CCD 9 by next transfer operation of the storage section 23, and then read out of the output section 10.
After signal charges associated with an image have been all read out through the horizontal CCD 9, the drive signals 24 applied to the image section 22 and the drive signals 25 applied to the storage section 23 become high-speed transfer pulses again, and then the next signal charges are transferred to the storage section 23. In the same manner as mentioned above, signal charges associated with an image are successively read out pixel by pixel.
TDI operation CCD image sensors and full-frame type CCD image sensors basically have the same structure as that of the above-mentioned frame transfer type CCD image sensor except that the storage section 23 is eliminated. A TDI operation CCD image sensor scans images incident thereon at a constant rate, and synchronizes transfer rate of signal charges with motion of the images to thereby concurrently carry out accumulation and transfer of signals, and provide high sensitivity. A full-frame type CCD image sensor includes a mechanical shutter in front of the CCD image sensor in order to shield the CCD image sensor from light when signal charges are read out of the CCD image sensor. When a charge accumulation period has expired, the mechanical shutter is closed. Then, signal charges are read out of the CCD image sensor for frame transfer in the same manner as signal charges are read out of the storage section of the above-mentioned frame transfer type CCD image sensor.
In the above-mentioned conventional solid-state image sensors including an electrode in a photoelectric conversion region, and a photodetector for detecting light passing through the electrode, such as a one-dimensional CCD, a CID image sensor, a TDI operation CCD image sensor, a frame transfer type CCD image sensor, and a full-frame type CCD image sensor, the gate electrode formed in a photoelectric conversion region and the CCD transfer electrode are composed of polysilicon. Hence, the conventional solid-state image sensors are accompanied with a problem that a photodetector has low quantum efficiency because of intensive absorption of light into the polysilicon electrode. In particular, a quantum efficiency is just a few percent in the blue region.
As a solution to the above-mentioned problem, various solid-state image sensors have been suggested in the followings.
1. Dale M. Brown et al., "Transparent Metal Oxide Electrode CID Imager", IEEE Journal of Solid-State Circuits, Vol. SC-11, No. 1, February 1976, pp. 128-132
2. David H. McCann et al., "Buried-Channel CCD Imaging Arrays with Tin-Oxide Transparent Gates", IEEE International Solid-State Circuits Conference Digest of Technical Papers, February 1978, pp. 30-31 and pp. 261-262
3. W. F. Keenan et al., "A Tin Oxide Transparent-Gate Buried-Channel Virtual-Phase CCD Imager", IEEE Transactions on Electron Devices, Vol. ED-32, No. 8, August 1985, pp. 1531-1533
The device suggested by D. M. Brown et al. is a CID image sensor. A structure of a unit pixel in the suggested CID image sensor is illustrated in FIG. 5. A thick field oxide film 30 is formed on an n-type silicon substrate 1. A portion of the field oxide film 30 in a region which will make an active region (namely, a recessed region inside a boundary 31 between a gate oxide film and the field oxide film 30) is removed, and a thin gate oxide film is formed in the region. In the region, a polysilicon low line (a horizontal selection line) 28 is first horizontally formed, and then a transparent electrode column line (a vertical selection line) 29 is formed on the polysilicon low line 28. The transparent electrode is composed of ITO (Indium-Tin Oxide: In.sub.2 O.sub.3 --SnO.sub.2), that is, indium oxide containing tin oxide at 5 to 10%.
A portion of the polysilicon low line (horizontal selection line) 28 covering the gate oxide film therewith constitutes a first gate electrode, and a portion of the transparent electrode column line (vertical selection line) 29 covering the gate oxide film therewith constitutes a second gate electrode. Though the polysilicon low line (horizontal selection line) 28 absorbs a light to much degree, optical loss in the transparent electrode column line (vertical selection line) 29 is small. Thus, a quantum efficiency is enhanced by a light incident on the device through the transparent electrode column line 29.
Devices suggested by D. H. McCann et al. are a one-dimensional CCD, A TDI operation CCD image sensor, and a frame transfer type CCD image sensor. FIG. 6 illustrates a cross-sectional structure of a light-receiving region of those devices. A p-type buried channel 32 is formed in the vicinity of a surface of an n-type silicon substrate 1, and a two-layered gate insulating film comprising a thermal oxide film 33 and a silicon nitride film 34 is formed on the silicon substrate 1. A plurality of first tin dioxide electrodes 35 is formed on the silicon nitride film 34 in such a way that the adjacent electrodes 35 are spaced away from each other. A first SILOX (SiO deposited by atmospheric pressure CVD. One of hyalines) layer 36 as an insulating film covers the first tin dioxide electrodes 35 therewith. A plurality of second tin dioxide electrodes 37 is formed on the first SILOX layer 36 so that each of the second tin dioxide electrodes 37 is disposed above a space formed between the adjacent first tin dioxide electrodes 35. The second tin dioxide electrodes 37 and exposed portions of the first SILOX layer 36 are covered with a second SILOX layer 38, on which an internal wiring composed of aluminum is formed in a non-illustrated region. On the second SILOX layer 38 are deposited a third SILOX layer 39 and a fourth SILOX layer 40. A light shield composed of aluminum for defining a light receiving region is formed on the fourth SILOX layer 40 in a non-illustrated region.
Since all CCD transfer electrodes in the light receiving region are composed of tin dioxide (SnO.sub.2), which is known as a transparent conductive material, the device suggested by D. H. McCann has a significantly enhanced quantum efficiency.
A device suggested by W. F. Keenan is a kind of a full-frame type CCD image sensor, but has a unique structure. This is a CCD operable by a single drive signal, and is called a virtual-phase (VP) CCD. FIG. 7 illustrates a cross-sectional structure of the suggested device. The device includes a silicon substrate 40, a gate oxide film 46 formed on the substrate 40, a plurality of tin oxide electrodes 42 formed on the gate oxide film 46 and spaced away from each other, and an oxide film 43 covering the tin oxide electrodes 42 and exposed portions of the gate oxide film 46 therewith.
As illustrated in FIG. 7, there is no vertical CCD electrode on the gate oxide film 46 between the tin oxide electrodes 42 which are driven with one drive signal. Instead of a vertical CCD electrode, boron virtual electrodes 41 are formed in the substrate 40 for keeping a surface potential in the substrate 40. The region where the boron virtual electrode 41 exists is a virtual-phase region 44, and the region where the tin oxide electrode 42 exists is a clock-phase region 45.
Though not illustrated, n-type buried channels are formed below the boron virtual electrodes 41 and the tin oxide electrodes 42. In n-type buried channels of both the virtual-phase 44 and the clock-phase 45, there are established impurities profiles which generate a difference in potential for forming both a barrier region and a storage region. Though a virtual-phase CCD is inferior in charge transfer capacity to an ordinary CCD which forms a barrier region and a storage section in CCD channel by drive signals to be applied to vertical CCD electrodes, a virtual-phase CCD provides an advantage of high quantum efficiency, because no vertical CCD electrode exists above the virtual-phase region 44. By composing vertical CCD electrodes formed on the clock-phase regions 45 of tin dioxide (SnO.sub.2) in place of polysilicon of which a vertical CCD electrode in a conventional virtual-phase CCD is composed, quantum efficiency is further enhanced, and uniformity in sensitivity between pixels is also enhanced.
As mentioned above, it is possible to improve quantum efficiency of a photodetector, in particular, in the blue region, by composing a gate electrode and CCD transfer electrode in a photoelectric conversion region of ITO (In.sub.2 O.sub.3 --SnO.sub.2) or tin oxide (SnO.sub.2). However, this is accompanied with the following problems.
Indium (In) which is a major constituent of ITO and tin (Sn) which is a major constituent of tin dioxide are materials both forming all ratio solid solution along with silicon, and hence do not form a stable form. In addition, oxides of indium and tin do not have sufficiently high thermal resistance. Hence, indium and tin tend to become contaminated, and thus pose a problem that limits the fabrication process, designability, performance limits, and productivity thereof are reduced in order to avoid deterioration of performances in a device.
In particular, thermal steps are limited. As described in the report of D. M. Brown, the maximum allowable temperature to be applied to a device is about 600.degree. C., if ITO or tin oxide were employed for forming an electrode. It would be difficult to implant and activate impurities, if an electrode were composed of ITO or tin oxide, and accordingly, it is not possible to employ self-align techniques useful for fabricating a smaller device and further for enabling a device to have higher performances. Furthermore, when an electrode is to be formed of a multi-layered structure, as having been reported by D. H. McCann, an insulating film between electrodes has to be a film which is capable of being formed at a low temperature process which would provide only low insulation. For instance, if an insulating film were to be formed of SILOX, it had to be formed at 400.degree. C. to 450.degree. C.
Japanese Unexamined Patent Publication No. 62-277878 has suggested a solid-state image sensor comprising a CCD type solid-state image sensor, and a micro color film disposed on the solid-state image sensor. The suggested CCD type solid-state image sensor is used for a camera, in particular, an electronic still camera, and is characterized by having means for preventing a light from entering the mage sensor.
Japanese Unexamined Patent Publication No. 63-500412, which is based on the international application numbered PCT/US86/01507 or assigned U.S. patent application Ser. No. 762,172, has suggested a frame transfer type CCD area image sensor having an improved horizontal resolution.